This invention concerns apparatus and methods for testing the strength of a bond in a semi-conductor device, and more particularly the strength of a bond between a substrate and a means of electrical connection thereto, typically a part-spherical deposit. Such deposits can be of solder, gold or other materials and are sometimes referred to as solder bumps or ball grid arrays.
Semiconductor devices are very small, typically from 0.2 mm square to 25 mm square. These devices have sites for the bonding of electrical conductors thereto. Sites typically comprise part spherical electrically conductive deposits of for example gold or solder, collectively known as balls, which in use have the appearance of a squashed sphere or low circular dome, and a diameter in the range 50-1000 μm. These deposits form part of the electrical path between, for example, a printed circuit board and a chip, and may directly connect components, or may be joined to a conductor which is itself connected to another component. Many such balls may be provided as a regular grid-like array on a substrate.
Discrete balls are typically applied to a substrate and reflowed during subsequent connection to another component.
It is necessary to test the mechanical strength of the bond between the ball deposit and the substrate in order to give confidence that the production bonding method is adequate, and that the bond strength is sufficient. One kind of test applies a shear load to the ball deposit by means of a shear tool of a shear test machine in which the substrate is secured. Another kind of test applies a pulling load to the ball deposit by means of a gripper of a pull test machine to which the substrate is secured.
There are two principal types of failure modes which occur during these types of test: ductile failure mode and brittle failure mode. Breaking a strong bond will result in ductile failure mode of the ball deposit, with progressive deformation of the ball deposit until the ball deposit is broken with part of the ball deposit often remaining adhered to the substrate. Breaking a weak bond will typically result in brittle failure mode with the ball deposit more or less cleanly tearing away from the substrate, leaving little residue adhered thereto. Consequently, inspection of the substrate after such a test can often indicate the mode of failure. It would be advantageous to provide test machine users with a test machine which could monitor parameters of the test tool and process those parameters to indicate automatically to the user the nature of the failure mode, and particularly whether a ductile or brittle failure mode. These “principle” failure modes can be subdivided into many other classifications. For example a brittle fracture can occur at the bond between the ball deposit and the conductive pad it is adhered to or between the conductive pad and the substrate that it is mounted to.
A known shear test apparatus comprises a machine having a support surface and a test head movable in a controlled manner relative to the support surface. The test head carries a cartridge specific to the test to be performed and having one of several interchangeable shear tools thereon. Typically the shear tool will be sized and/or shaped to suit the ball deposit to be tested. In use, the substrate to be tested is attached to the support surface of the machine, and the tool is mounted into the cartridge and driven against the ball deposit to perform the required test, which may be for example a shear test or a reciprocating fatigue test. Typically the tool moves against a stationary deposit.
A known pull test machine is similar and carries a cartridge having a gripper adapted to the size and shape of the ball deposit to be tested and operable to exert a pulling load on the ball deposit substantially orthogonally to the substrate.
Although shear tests and pull tests are somewhat different, the force/displacement characteristics are similar and can be used to classify the failure mode, particularly whether the failure mode is ductile or brittle.
A typical tool is small. The cartridge upon which the tool is secured has a flexible element on which is mounted one or more force gauges (such as strain gauges). Thus, the force between the tool and ball deposit is measured at a distance by deflection in the flexible elements of the cartridge. WO-A-2005/114722 shows an example of such a cartridge. The cartridge shown in WO-A-2005/114722 can be used in the Dage Model 4000 Series machine available from Dage Precision Industries Ltd., of Aylesbury, United Kingdom. This machine typically measures the peak force necessary to break a ball deposit off a substrate. Although the machine is able to measure peak force, it is difficult to determine whether the bond between the ball and substrate has failed as a result of ductile failure mode or brittle failure mode based on measurement of peak force alone. This difficulty arises from the fact that the peak forces in both brittle and ductile failure modes may be broadly similar for a given size and shape of ball deposit.
It would be desirable to provide a method and apparatus for distinguishing the mode of bond failure, and particularly to distinguish between the brittle and ductile failure modes and any other modes that these principle modes are subdivided into. As mentioned above, if breaking a ball off a substrate produces a ductile failure mode, that indicates that the bond between the ball and the substrate is a good bond, whereas a brittle failure mode would indicate that the bond is suspect and may be poor. Thus, providing the user of the machine with an output displayed on the machine monitor, for example, which indicates to the user whether the failure mode was ductile or brittle, indicates to the user whether the bond tested was a good bond or a poor bond.